A light receiving module for optical communication in the related art is illustrated in FIG. 1. Laser light emitted from an optical fiber illustrated in FIG. 1 has the features of emissive light emitted at a predetermined angle. In order to effectively receive such emissive light, there is a need for a lens that can converge laser light with a photodiode for receiving light, and in the related art, light receiving elements included a lens, so the light receiving elements and an optical fiber were optically aligned to receive light. Further, bidirectional light transmitting/receiving modules in the related art, as illustrated in FIG. 2, were manufactured to send signals to an optical fiber, using an assembly of a TO (Transistor outline) type light transmitting element equipped with a lens on the outer side and a TO type light receiving element equipped with a lens. In general, light transmitting elements include a semiconductor laser diode chip, which emits light in the type of emissive light having a predetermined emission angle. Accordingly, in order to collect light emitted from a laser diode chip, which emits light in the type of emissive light having a predetermined emission angle, to an optical fiber, there is a need for a lens that converts emissive light into converging light between a laser diode chip and an optical fiber, so light transmitting elements, as in FIG. 2, is equipped with a lens and converts laser light, which is emitted from a laser diode chip therein, into converging light and optically coupling it to an optical fiber. Laser light for receiving is emitted from an optical module for bidirectional communication to the same optical fiber that receives light for transmitting. Laser light emitted from an optical fiber is also emissive light having a predetermined emission angle, so the size of the laser light increases in proportion to the traveling distance of the light. Photodiodes in optical receiving elements are usually very small in dozens of nanometers, so there is a need for a lens for converging laser light having the features of emissive light from an optical fiber to a photodiode, as in FIG. 2, in order to converge light emitted from an optical fiber to those very small photodiodes.
As illustrated in FIGS. 1 and 2, in common light receiving modules or common bidirectional optical communication modules, a lens is included in a light receiving element, so laser light having the features of emissive light and emitted from an optical fiber converges into a light receiving area of a photodiode in the light receiving element. However, laser light travels at different angles, depending on areas, between the lens and the light receiving area of the photodiode. This is illustrated in FIG. 3. That is, laser light from, an optical, fiber 600 becomes converging light that converges to a light receiving area 211 of a photodiode 210 through a lens 410 in a light receiving element 20.
Recently, a communication standard of NG-PON2 (Next Generation Passive optical Network 2) has been established by ITU (International Telecommunication Union), in which a wavelength, multiplying method using one optical fiber using four channel wavelengths has been employed, so the communication standard requires a wavelength-tunable light receiving element that can tune and selectively receive specific wavelengths. Tuning and selectively receiving wavelengths can be easily achieved by mounting a wavelength-tunable wavelength-selective filter, which can tune a wavelength, between a lens and a photodiode in a light receiving element. However, when light having a specific wavelength is selectively transmitted using a wavelength-tunable filter, the wavelength of the transmitted light depends on the incident angle on the wavelength-selective filter. Accordingly, in order to effectively select and transmit a specific wavelength using a wavelength-selective filter, the light traveling into the wavelength-selective filter preferably have the features of parallel light, as illustrated in FIG. 4, rather than the features of converging light, as illustrated in FIG. 3.
Further, as illustrated in FIG. 5, it is possible to convert light from an optical fiber into parallel light by attaching a lens to a light receiving element, but conversion of emissive light into parallel light depends on optical arrangement of an optical fiber 600 and a light receiving element 22. That is, as in FIG. 5, when laser light traveling into the light receiving element 22 by optical arrangement between the optical fiber 600 and the light receiving element 22 is converted into the parallel light, the optical fiber 600 should be accurately aligned with the focus of a lens 225, but when the optical fiber 600 is out of the focus of the lens 225, light passing through the lens 225 cannot be parallel light. However, when optical arrangement is achieved, as described above, the features of parallel light, converging light, and emissive light that laser light has in the light receiving element 22 depend on the degree of the optical arrangement, and a change in convergence of light is a phenomenon generated in the light receiving element 22, so it is difficult to know whether the laser light becomes parallel light in the light receiving element 22 in optical arrangement.
Accordingly, it is very difficult to convert light having the features of emissive light outside a light receiving element into parallel light inside the light receiving element, using a lens attached to the light receiving element in the light receiving module illustrated in FIG. 1 and the optical module for bidirectional communication illustrated in FIG. 2. Therefore, when the light receiving element is a wavelength-tunable light receiving element, it is difficult to make the laser light traveling into a wavelength-tunable selective filter into parallel light, so wavelength selectivity is low.